ATP World Tour Finals 2013: Novak Djokovic chases Rafael Nadal all the way with victory over Juan Martin del Potro

Serb follows Spaniard into last four at the O2 Arena

Roger Federer enjoyed a routine victory over Richard Gasquet at the O2 in London (Reuters)

The chase for the year-end world No 1 ranking may be over but Novak Djokovic continues to snap at Rafael Nadal’s heels. Twenty-four hours after Nadal became the first player to secure his place in the last four here at the Barclays ATP World Tour Finals, Djokovic followed in his rival’s footsteps by beating Juan Martin del Potro 6-3, 3-6, 6-3.

With Roger Federer also taking a step towards qualification for the semi-finals by beating Richard Gasquet 6-4, 6-3, these end-of-year championships could be heading for a fitting finale. Ever since losing to Nadal in the US Open final two months ago Djokovic has been relentless in his pursuit of the Spaniard, who only ensured that he would end the year on top of the world rankings with his second win here on Wednesday.

This was Djokovic’s 19th successive victory since that defeat in New York. He has won three tournaments in that run and is going all out to defend his title here and go on to win the Davis Cup for Serbia in Prague next weekend.

Del Potro has been in similarly outstanding form, winning 16 of his 18 matches since the US Open, and the 25-year-old Argentine pushed Djokovic hard with his second-set fightback. However, he has now lost seven of his last eight meetings with the world No 2, who made his decisive move in the final set when he broke to love in the sixth game after his opponent made two successive forehand errors. Having lost his way in the second set, Djokovic was a model of consistency in the decider.

Del Potro can still qualify for the semi-finals if he beats Federer in a showdown on Saturday. Federer bounced back from his defeat by Djokovic to beat Gasquet with something to spare, keeping alive his hopes of winning this title for the seventh time.

After the match Federer was asked for his views on the sport’s drug-testing procedures, Djokovic having said earlier in the week that he had lost faith in the system after the 12-month suspension handed to Viktor Troicki, his friend and fellow Serb, for missing a blood test. Troicki had claimed that a doping officer told him he could return for a test the following day because he was feeling unwell. The former world No 12, whose urine test the same day was negative, returned 24 hours later for a blood test, which was negative.

Djokovic was critical of the doctor, but Federer, said: “Overall I trust the system. I think they’re all very professional. When you are requested for a sample, you have to give the sample. It doesn’t matter how bad you feel. I’m sorry. The test the next day is not a test any more, because of what could have happened overnight.”